1. Field of the Invention
The present invention relates to an image forming device that makes it hard for toner to adhere to an idling photosensitive drum after the image forming device is powered off.
2. Description of the Related Art
Typical laser printers, copiers and other image forming devices include a photosensitive drum (electrostatic latent image bearing member) having a charge generating layer and a charge transport layer on a base material layer. A corona discharge or other process is performed to pre-charge the photosensitive drum and form an electrostatic latent image on the photosensitive drum by exposing the photosensitive drum to laser, LED or other types of light. The electrostatic latent image is made visible through the use of toner or other developing agent. The toner image is transferred to paper or other recording medium and thermally fixed by a thermal fixing device.
Generally, the photosensitive drum is a cylindrical shape and rotates around an axis extending in a direction perpendicular to the sheet feeding direction. The photosensitive drum is rotated by drive power transmitted from a drive source via gears or other drive power transmission mechanism. A charging unit, an exposure unit that exposes the drum to light, a developing unit that develops electrostatic latent image formed on the drum to a visible image using toner, and a transfer unit that transfers the toner image onto a recording medium are disposed around the photosensitive drum in confronting relation with the outer periphery of the drum.
If the gear reduction ratio of the drive power transmission mechanism interposed between the drive source and the photosensitive drum is increased for the sake of making a more compact image forming device and saving energy, the load on the photosensitive drum is lowered. This increases photosensitive drum idling when transmission of driving force to the photosensitive drum ends after forming the image. If, for example, positively charged toner is used, the surface of the photosensitive drum whose potential has been lowered by the transfer unit will reach the nip area (the area of actual contact between the photosensitive drum and the developing unit) through the idling of the photosensitive drum causing toner to move from the developing unit to the photosensitive unit. The toner that adheres to the photosensitive drum moves to the transfer unit, which is in contact with the photosensitive drum, at next printing soiling the rear side of the paper during printing.
The application of the charging bias by the charging unit is continued until the photosensitive drum stops completely allowing the charging unit to control the potential relationship between the idling photosensitive drum and the developing unit. This means that the nip area of the photosensitive drum 27 has a higher potential than the developing unit preventing toner from the developing unit to adhere to the photosensitive drum as disclosed in Japanese Patent Application Publication No. 62-201470 and 6-214442.
The above patent application stipulates that charging by the charging unit be continued until the photosensitive drum stops. This means that the photosensitive drum is partially charged in the time period immediately before the photosensitive drum stops and until the photosensitive drum stops, which reduces the service life of the photosensitive drum.